Maria Catharina van Grunsven (6 September 1918 – 1 March 2004), also known as Riet van Grunsven or Riet van de Haterd-van Grunsven, was an armed member of the Dutch Resistance during World War II. One of 21 people who became “line crossers,” special agents who helped to maintain a secret connection between occupied and liberated Netherlands across the waterways of the Biesbosch and the Merwede, she operated as a courier for part of the larger Biesbosch resistance group, and the Albrecht intelligence group. During this resistance work, she was given the aliases “Ice Block” and “Trouble.”